Seismic Swarm SVS20210927.1 Recorded South of Mammoth, Wyoming

On 27 September 2021, a seismic swarm designated SVS20210927.1 was detected approximately 28 km south of Mammoth, Wyoming. The sequence began at 02:48 and concluded at 10:19, encompassing 26 earthquakes over 7 hours and 30 minutes. Magnitudes ranged from -0.7 to 1.7, with focal depths between 2 km and 10 km. The largest events reached magnitude 1.7 at 03:11:52 and again at 04:17:28.

Events clustered primarily in the first two hours, followed by sporadic activity through mid-morning. Shallow depths dominated the early phase, with many hypocenters at 2–5 km, while later shocks extended slightly deeper. Negative magnitudes indicate microseismicity typical of fluid-driven swarms rather than tectonic mainshock-aftershock sequences.

The swarm occurred within the Yellowstone volcanic field, a region underlain by a continental hotspot that has produced three major caldera-forming eruptions over the past 2.1 million years. Ongoing magma intrusion, hydrothermal fluid circulation, and crustal extension generate persistent low-level seismicity. Mammoth lies near the northern margin of the 640,000-year-old Yellowstone Caldera, where fault zones and fractured volcanic rocks facilitate swarm activity.

Since 2000, 65 swarms have been documented in the broader area. Annual counts show variability, with peaks of nine events in 2000 and six each in 2008, 2014, and 2020. The 2021 total reached two swarms by late September, consistent with the long-term average of roughly three per year.

Such swarms typically reflect migration of hydrothermal fluids or minor magmatic movement rather than precursory signals of larger eruptions. Monitoring networks operated by the U.S. Geological Survey and the University of Utah Seismograph Stations track these episodes to refine hazard assessments for the Yellowstone region.

References

• U.S. Geological Survey, Yellowstone Volcano Observatory reports (2021)
• University of Utah Seismograph Stations, Intermountain West earthquake catalog
• Yellowstone National Park geologic resources inventory (updated 2023)